The invention relates to a high pressure mercury vapor discharge lamp. The lamp comprises a discharge vessel of light-transmissive material having high strength at high temperatures, and electrodes of refractory material sealed into the discharge vessel. It is filled with mercury as a buffer gas, an inert gas as the ignition gas, and at least one emitting metal halide and at least one further metal halide.
High pressure mercury vapor discharge lamps which contain metal halide additives are known. DT-PS 1 184 008, for instance, discloses the halides of metals of Group I - III of the Periodic Table as such additives. U.S. Pat. No. 3,654,506 discloses the halides of the rare-earth metals as such additives. U.S. Pat. No. 3,753,018 discloses as additives, the iodides of sodium, lithium, cadmium, thallium, indium, tin, dysprosium and scandium, and preferably the combination of sodium, thallium and indium. Canadian patent No. 936,907 discloses as the emitting substance, the iodides of thallium, scandium, calcium, cesium, dysprosium, sodium, samarium or tin, lanthanum, lithium and barium, and as a non-emitting buffer substance the iodides of antimony, arsenic, bismuth, indium, zinc, cadmium and lead. The purpose of these additives to the mercury, which are excited to luminosity, is to bring about in the aforesaid lamps as white a light emission as possible and a high luminous efficacy. Lamps with additives which predominantly emit radiation of the resonant lines may have high or low color temperatures, but color rendering is unsatisfactory in most cases (R.sub.a is low). Lamps containing rare-earth metal additives, on the other hand, display a multiline spectrum. They have a high color temperature of about 6000 K together with the high luminous efficacy of more than 70 lm/W, and a good color rendering (R.sub.a is high). Moreover, it is well known that lamps containing tin halide additives to the mercury, which are excited only to luminosity, display continuous molecular radiation with a predominantly low color temperature of about 4000-5000 K and good color rendering of R.sub.a greater than 85, but that the luminous efficacy of about 50 lm/W is extremely low and insufficient for a large variety of uses (U.S. Pat. No. 3,566,178).
It is an object of the present invention to provide a lamp which in contradistinction to the aforesaid has the advantageous combination of a low color temperature, and at the same time a high luminous efficacy and good color rendering, namely, a high color rendering index R.sub.a.